Likely a big job to reclaim. Hire help.

If the grass is growing within the lavender plant, that seems like a hand weeding job to untangle the two.

For grass/weeds growing in aisles/space between plants, one option is to hoe or weed whack and then install long black plastic strips or cardboard to use solar heat to kill the grass/weeds in 2-3 months. Keep an eye on it as you don‘t want the heat spreading to the lavender’s soil/roots.

Please post a photo of the rejuvenated lavender fields when in bloom. It will be spectacular!

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Design Fan gave excellent advice applicable in a lot of geographical locations.

You can often get free advice from your state's farm bureau or land grant university extension service.

The kinds of weed-reduction you can do depend on your location, climate and soil.

What kind of farming experience do you already have? Any equipment acquired along with the land? Can you hire seasonal help?